It's one of the earliest lesson most painters – be they professional or weekend home warriors – learn soon after popping open their first paint can: How to thin paint that is too thick and is "fighting" efforts to spread it evenly over a surface. Three tenets should guide your efforts. One, double-check the type of paint you're working with. Homemade paint thinners that are appropriate for oil-based paint are not suited for latex paint – and vice versa. Two, use a light touch to thin the paint. Just like modifying a too-thick barbecue sauce in the kitchen, it's easier to add a thinning agent than to "thicken up" a runny one. Three, watch your ratio of paint to thinner. Paint is a forgiving substance, but still, the ratios should be close. If you can follow these three tenets, you'll find smooth sailing -- and even smoother painting -- right around the corner.
Thin Water-based Latex Paint
Turn to water -- right from the faucet -- to thin latex paint. Abide by a ratio of ½ cup of room-temperature water for each 1 gallon of paint. Remember that you can always add more water if the paint is stubbornly thick.
Allow the paint to come to room temperature, if it isn't already. Cold paint can appear thicker than it actually is, and if you rush this job, you could end up using too much water to thin it.
Pour about half the paint into a paint bucket. Add about half of the correct proportion of water, followed by the rest of the paint and then the rest of the water.
Stir the paint thoroughly. Ensure that all the water blends with the paint and does not form a puddle on the surface. Let the paint rest for a few minutes and then stir it again. Ideally, the paint should be thick enough that it adheres to the stick -- this consistency will ensure coverage -- while still dripping off into the bucket. The paint should not be runny.
Dip a paint brush into the thinned paint and test it on a piece of poster board – a go-to painting tool for any painter. To make the most accurate observation, let the paint dry before inspecting it. If the paint is still too thick, add a few drops of water, stir the paint and repeat, if necessary, until you achieve the right consistency.
Thin Oil-based Paint
Turn to mineral spirits or acetone to thin oil-based paint. Mineral spirits is a petroleum-based product that has become a favored substitute for turpentine. Acetone -- a moderately aggressive solvent -- is the active ingredient in nail polish remover. Both products can be found on the shelves of most home improvement stores.
Set up your painting project in a well ventilated area. The odor from mineral spirits and acetone is not as harsh as that from turpentine, but it's still detectable. Protect yourself by putting on a face mask and gloves.
Strive for a beginning ratio of three parts paint to one part thinner, knowing that you can always add one more thinner if need be. Pour about half the paint into a paint bucket. Add about half of the correct proportion of thinner, followed by the rest of the paint and then the rest of the thinner.
Stir the paint thoroughly. Let the paint rest for a few minutes and then stir it again. Ideally, the paint should be thick enough that it adheres to the stick while still dripping off into the bucket.
Dip a paint brush into the thinned paint and test it on a piece of poster board. To make the most accurate observation, let the paint dry before inspecting it. Check the drying time on the paint can; unless you found a "fast-drying" variety, oil-based paint usually takes longer to dry than latex paint. Add a few more drops of thinner if the paint is still too thick.